Amplification in optical fiber transmission systems using rare earth doped optical fiber amplifiers, such as, for example, erbium doped fiber amplifiers (EDFA) has been implemented widely due to the advantageous economics and wideband multi-channel operation of such amplifiers. In such an optical transmission system, inter-channel stimulated Raman scattering (SRS) may result in a tilted gain characteristic across the wavelength division multiplexed (WDM) channels (denoted as Raman tilt hereafter) by externally supplied Raman pump radiation using the Raman pumps to add Raman gain. Tilt is the well known transmission impairment characterized by increased power consumption at decreasing wavelength (increasing frequency). Without compensation for tilt, such an effect accumulates span by span and results in serious optical signal to noise ratio (OSNR) degradation in the shorter-wavelength channels and serious nonlinear penalty in the longer-wavelength channels. In a traditional point-to-point WDM system, it is known to compensate for Raman tilt due to inter-channel SRS in the transmission fiber by adding a static tilt compensator after every span. But optical communication is evolving from current point-to-point systems (in which there are no intermediate add/drop points) to dynamic optical networks, in which channels will be added and dropped at intermediate points in the end-to-end system by using, for example, known remotely reconfigurable optical add/drop multiplexers (ROADM) to meet the varying capacity demands. A typical 80 channel C-band WDM system using a single mode fiber as the transmission fiber of, for example, a span length of twenty spans can contribute 1 dB of tilt per span across C-band or a cumulative 20 dB of Raman tilt will be present in such a long haul system. (Raman tilt can be even higher when nonzero-dispersion shifted fibers are used as the transmission fibers).
FIG. 1 is an illustration of an exemplary fiber optic amplifier system having a transmission fiber (Trans. Fiber) 100-1 followed by a first Erbium doped fiber amplifier (EDFA). In the drawings, similar reference numerals will be used to denote similar elements and the first number of a given reference numeral indicates the number of the figure where that element first appears. A dispersion compensation fiber (DCF) 102 separates the first EDFA 101-1 from a second Erbium doped fiber optic amplifier 101-2. A reconfigurable optical add/drop multiplexer (ROADM) 103 is shown following the second EDFA 101-2 for adding and dropping channels. Adding and dropping channels causes transient events, for example, tilt transients. These events, if left uncompensated, create transmission degradation. Depending on the wavelengths dropped and added, a fast transient event occurs, typically, in the form of a transmission degrading tilt transient. Referring to FIG. 1, also, accidental loss of channels due to a transmission fiber cut or a component failure of one of the components such as an EDFA or the DCF 102 in front of an ROADM 103 will also lead to a sudden change in channel count and transient events such as tilt transients. Therefore, there will be a resulting change of the overall optical power in a link following an ROADM node. Of course, the purpose of an ROADM is to add and to drop channels which can likewise result in a change of overall optical power. The strong reduction/increase of total launch power into the transmission fiber section (Trans. Fiber) 100-2 following the ROADM 103 will result, for example, in a substantial reduction/increase of Raman tilt.
As is shown in FIG. 1, the respective channel patterns 1 and 2 shown after ROADM 103 will exhibit tilt in their spectral powers across their bandwidth as represented by the same channel patterns depicted at the output of Trans. Fiber 100-2. The depicted tilt is represented as a smooth sloping line as a general case to show Raman tilt. In the case of co-directional propagation of the signal channels, the transition time of the induced transients are equal to the transition time of the switching events, which can be very fast. As a result, an optical transmission system with fast Raman tilt transients control capability is needed for operating in a dynamic optical network. A “tilt transient” is not shown by way of example in FIG. 2(b) where the composite Raman gain profile results in a tilted profile but, depending on the dropped or added wavelengths or the cause of a transient event, the transient event may cause bumps (overshoots and undershoots) in the smooth tilt gain characteristic and thus may be described generally as a transient event and is not one as depicted that specifically results in a smoothly tilted gain profile.
Recently, P. M. Krummrich and inventor Martin Birk, in their article “Compensation of Raman Transients in Optical Networks,” presented at OFC 2004, paper MF 82 and their article, “Experimental Investigation of Compensation of Raman-induced Power Transients from WDM Channel Interactions,” IEEE Photonics Technology Letters, Vol. 17, no. 5, pp 1094-96, May, 2005, suggest adding a standalone dynamic tilt compensator after every span or after a small number of spans into a traditional EDFA system to deal with this problem. In their method, the dynamic tilt compensator is based on a periodically poled LiNbO3 technology with a two-stage design. But such a solution adds considerable cost to the cost of a long haul transmission fiber optic system because the fast dynamic tilt compensator itself is quite expensive. Moreover, one dynamic tilt filter may introduce more than 5 dB of insertion loss into the system. Therefore, the insertion loss of the device will require additional amplification to compensate for the insertion loss which, if the amplification is necessary, will add to the overall system cost.
Also, recently, the inventors have prepared and are filing a number of patent applications directed to dynamic gain control for a fiber optic system as represented by U.S. patent application Ser. Nos. 11/273,868 and 11/274,666 filed Nov. 15, 2005; U.S. patent application Ser. No. 11/424,306 filed and U.S. patent application Ser. No. 11/381,244, filed May 2, 2006, all incorporated by reference as to their entire contents. For example, the inventors propose the use of optical amplifiers which are either forward or reverse pumped, RFA's comprising a plurality of Raman pumps that may be controlled by a single control circuit and feed forward and feed backward control circuits and equations and algorithms for their control in combination or used separately. In the specification and claims, a forward Raman pump is a power source that provides power to a signal by a co-propagating signal-pump Raman interaction and a backward Raman pump is a power source that provides power to a signal by counter-propagating signal-pump Raman interaction. A Raman fiber amplifier, either forward or reverse, and an associated dynamic gain control circuit can inherently provide transient tilt control because of their inherent speed.
Apart from a pure EDFA system (no Raman amplification), a hybrid (or combined) end-to-end fiber optical system which includes both EDFA and Raman amplifiers in the system or an all-Raman system have also been widely investigated in recent years. In a known hybrid EDFA/Raman optical system, both EDFA and Raman fiber amplifiers (RFA) (distributed) are used in many or even every span. In an all-Raman system, RFAs (distributed and discrete) are the only optical amplifiers used. In both systems, externally supplied Raman pumps are fed into the transmission fiber 100 at every span; consequently, the signal experiences Raman gain not only from the other signals input to the system but also from the Raman pumps. Because usually more than one Raman pump is needed to obtain a flat gain characteristic over a wide bandwidth and because the power conversion efficiency of a distributed RFA is typically lower than an EDFA, using a distributed RFA at every span results in a considerable cost increase compared to an EDFA only system. Such a cost penalty may outweigh the noise performance gain provided by using distributed RFAs at every span under some circumstances. Consequently, there remains a need in the art for a cost effective approach to the problem of transient event control, especially, tilt transients, due, for example, to channel addition and dropping in a long haul fiber optic system.